SELECT 
  cscart_product_prices.product_id, 
  MIN(
    IF(
      cscart_product_prices.percentage_discount = 0, 
      cscart_product_prices.price, 
      cscart_product_prices.price - (
        cscart_product_prices.price * cscart_product_prices.percentage_discount
      )/ 100
    )
  ) AS price 
FROM 
  cscart_product_prices 
WHERE 
  cscart_product_prices.product_id IN (
    6962, 6966, 6967, 6965, 6963, 6952, 6953, 
    6947, 6959, 3425, 6958, 6960, 6968, 
    6961, 6964, 6957, 6956, 6954, 6955, 
    6948, 6950, 6949, 6946, 6945, 3414, 
    6938, 6941, 6944, 3412, 6937, 6939, 
    6943, 6970, 6971, 6972, 6973, 6974, 
    6975
  ) 
  AND cscart_product_prices.lower_limit = 1 
  AND cscart_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  cscart_product_prices.product_id

Query time 0.00094

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "26.61"
    },
    "grouping_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "cscart_product_prices",
        "access_type": "range",
        "possible_keys": [
          "usergroup",
          "product_id",
          "lower_limit",
          "usergroup_id"
        ],
        "key": "product_id",
        "used_key_parts": [
          "product_id"
        ],
        "key_length": "3",
        "rows_examined_per_scan": 38,
        "rows_produced_per_join": 7,
        "filtered": "20.00",
        "index_condition": "(`gaseus`.`cscart_product_prices`.`product_id` in (6962,6966,6967,6965,6963,6952,6953,6947,6959,3425,6958,6960,6968,6961,6964,6957,6956,6954,6955,6948,6950,6949,6946,6945,3414,6938,6941,6944,3412,6937,6939,6943,6970,6971,6972,6973,6974,6975))",
        "cost_info": {
          "read_cost": "25.85",
          "eval_cost": "0.76",
          "prefix_cost": "26.61",
          "data_read_per_join": "182"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`gaseus`.`cscart_product_prices`.`lower_limit` = 1) and (`gaseus`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}